Day 6, actual day 4

A hack joke we've all heard a heard a thousand times, but here it is.  I went to water a plant on the front bedroom window ledge (that isn't the hack bit).  I could see a man slowly walking down the pavement past the house, he was faffing with an electronic device.  A meter reader.  I went downstairs to the front door and there was the card, "we called today but you were out", etc, etc.

Now, why is it that they knock at a frequency outside that of human hearing, with all the strength and power of an injured ant worried that it might be causing a disturbance?

Insert the name of your favoured delivery company in the above.

There isn't actually a joke above, or anywhere else.